1. Field of the Invention
This invention relates to measuring and testing electrochemical processes; and it relates particularly to instruments for the practice of electrochemical and electroanalytical techniques used in the study of corrosion processes in conductive media.
2. Description of the Prior Art
In the field of electrochemical and electroanalytical instruments and processes, many types of analyses can be performed in external systems which have voltage-current related properties. The study of corrosion phenomena is one such area. Other areas include, but are not limited to, the following: phase-sensitive AC, pulse, and DC polarography; anodic stripping analysis; cycling and potential sweep voltammetry; pH and specific ion measurement; direct potentiometry; controlled potential and controlled current electrolysis; chronopotentiometry; chronoamperometry; pulse response studies; electrical double layer capacitance measurements; itensiostatic, potentiostatic, and potentiokinetic methods for corrosion studies; and performing corrosion measurements as described in U.S. Pat. No. 3,101,406.
Instruments for practicing these analyses may be denoted, in a most general sense, as "potentiostats." Such instruments are arranged to produce and maintain a given voltage within the external system having voltage-current related properties by regulation of the current flowing therethrough. The potentiostatic instruments usually include a high impedance voltmeter for determining the maintained potential, a current source capable of maintaining a current flow to insure a constant value for the induced potential, and various auxiliary equipment which includes the cells, electrodes, and so forth, forming the external system, and various types of readout devices (ammeters, voltmeters, recorders, scopes, etc.). The auxiliary equipment can also include timers, recorders, and function generators capable of producing pulses, square waves, sawtooths and sine wave voltage sweeps. The readout means include oscilloscopes, various forms of wave analyzers, and impedance bridges.
The external system can be the classic types of electrochemical cells such as dropping mercury electrodes, hydrogen and glass reference electrodes, specific ion electrodes, metal electrodes and various combinations of such electrodes. These external systems all have a common characteristic at their electrical terminals. The external systems exhibit voltage-curent related properties at their terminals. In particular, a potential can be induced between a first pair of terminals, and other terminals are employed for passing a current through the cell which induces and maintains such potential. The magnitude and direction of the current flow and its function with time have a prescribed relationship to induced potential. These related properties of voltage and current are definitive of the electrochemical and electroanalytical composition of the cell.
The most common analysis of external systems having voltage-current related properties in aqueous media is voltammetry. In voltammetry, a pair of electrodes are employed for sensing the induced potential in the system. Other electrodes are employed for passing current through the conductive media for inducing the potential between the first electrode pair. The induced potential may be maintained constant for a given period of time, or it can be varied from a first, to a second, or even to a third, magnitude and varied at a constant rate with time, or with other functions with time such as exhibited by a sine wave or triangular wave.
Another electrochemical analysis of an external system found in measurements of corrosion phenomena is described in U.S. Pat. No. 3,406,101. In this patent, there is described an external system formed by a corrosion cell containing an aqueous corrodant in which are immersed three electrodes. Current is passed between two electrodes and induces a potential relative to a third electrode (reference). The current flow required to induce a certain potential change between the reference and one other electrode (test) is employed to determine the rate of corrosion which is occurring at the test electrode in the cell. Thus, the current flow in such a cell is the "readout" of the corrosion occurring at the test electrode.
The known external systems having voltage-current related properties have a plurality of terminals and conventionally have at least four terminals (e.g., two terminals to sense induced potential and two terminals to maintain current flow). For example, four-electrode conductivity cells are an external system having voltage-current related properties in which the potentiostatic instruments find ready application.
Prior art instruments employed in the electrochemical and electroanalytical field, particularly potentiostatic instruments, have provided useful results. However, these instruments left much to be desired in easy and reliable operation. First, the induced potential in the external system either had to be maintained at fixed levels for given lengths of time, and then changed with a square wave function to other levels in order to insure stable operation. Voltage sweeping has been attained, for the most part, by motor-driven rheostats which suffer from mechanical and electrical aberrations (i.e., nonlinear sweeing). In addition, should the voltage sweep direction of the induced potential be reversed, a time lag in voltage shift was experienced (i.e., discontinuous operation). A linear change in voltage within the external system is produced by a logarithmic change in current. Thus, a voltage shift of several tenths of a volt could change the current over several decades in magnitude. This linear-logarithmic property required complex switching equipment to insure even moderately accurate measurement in the magnitude of current flow. Furthermore, a third problem immediately arises. Since the data or readouts were in the linear voltage-amperage measurement system, correlating a certain voltage change to a certain current magnitude required a manual plot of volts and amperes upon log function graph paper or other such means. The voltage of the external system can be swept linearly over an extended range (0-10 volts) by the potentiostatic instrument. The current magnitude can change responsively over eight decades in less than 0.5 volts and is very difficult to obtain from linear data whose accuracy is good only for about four decades. Thus, the instrument operator was never sure that the voltage sweep information in his readout was directly correlatable to the related current magnitude. These operational difficulties in prior instruments have prevented the ready and accurate application of the electrochemical and electroanalytical techniques in evaluating corrosion phenomena, and other related analysis of external systems having voltage-current related properties. The present invention is directed towards an instrument which avoids these problems.